1. Field of Disclosed Subject Matter
This disclosure relates to systems and methods that provide an ability to deliver high quality variable pearlescent ink images on successive substrates using a proposed variable digital offset lithographic image forming architecture.
2. Related Art
The visual phenomenon known as pearlescence refers generally to a limitedly iridescent response of a prepared surface to incident light. Pearlescent surfaces reflect incident light in a manner that appears to change color like the surface of a pearl as the angle of view of the surface or the angle of illumination by the incident light on the surface changes. The broader iridescent visual phenomenon is commonly observed from surfaces including soap bubbles, butterfly wings, and sea shells.
Pearlescent coatings or pigments, which may be deliverable as paints or inks provide the pearlescent visual effect for decorative or ornamental purposes using printable products and/or industrial coatings or paints. A common contemporary example of the decorative use of the pearlescent visual phenomenon, and more broadly the iridescent visual phenomenon, is in its application in paint formulations used by the automotive industry to give cars a lustrous, metallic color changing (or chameleon-like) appearance.
Efforts to extend the concepts of delivering pearlescent images to surfaces in the printing arts have seen limited success. Printed pearlescent products have been explored not only for the decorative properties that the pearlescent printed materials provide, but also for their potential security properties in providing a capability to uniquely mark a document for identification in a manner that may be more difficult, for example, for a counterfeiter to reproduce.
The pearlescent effect on printed products can be difficult to generate because the metallic sheen is generated through the use of comparatively large solid pigment particles, on the order of tens of microns, which are difficult to stabilize in an ink composition. Digital printing using pearlescent inks is very difficult using conventional digital imaging techniques as the pearlescent inks prove nearly impossible to jet based on the necessary particle sizes appropriate to produce the pearlescent effect in the finished documents.
Pearlescent inks can be used to form printed materials using conventional lithographic and offset lithographic printing techniques. These techniques, however, use plates that are permanently patterned, and are, therefore, generally considered to be most useful only when printing a same image in long print runs. Conventional lithographic techniques, while suitable for producing pearlescent images on documents, are generally not considered amenable to creating and printing a new pattern from one page to the next because, according to known methods, removing and replacing of plates, including on a print cylinder, is required in order to change images. Because conventional lithographic techniques cannot accommodate true high speed variable data printing processes in which the images to be printed change from impression to impression, for example, as in the case of digital printing systems, these techniques do not provide an opportunity to effectively and/or economically produce pearlescent images on documents that change from page to page or across small runs of pages.
Typically, ink jet printing techniques are considered to be most amenable to high speed variable data digital image forming. A shortfall for certain printing applications, including pearlescent image forming, is that the physical compositions of the jetted inks must be carefully controlled within fairly rigid parameters. Generally, jetted inks cannot be particularly viscous and/or they cannot contain solid pigment particles over a certain size. Overly viscous ink compositions, or ink compositions having solid particles with larger particle sizes, will tend to very easily clog the jets and introduce other issues that adversely affect image forming operations through the ink jetting process supporting the digital printing. Producing jet nozzles with sufficient diameters to prevent this clogging is not reasonably feasible based on the array of other physical and operational characteristics that appropriately large nozzles would introduce.
The preparation of jettable inks often involves extensive pulverization of the solid elements, such as the pigments or other solids included in the ink compositions, with, for example, steel shot or in a grinder (high speed media mill), in order that the solid elements are more easily suspended in the liquid thus making them easier to pass in the ink jetting process.
A difficulty arises in the compatibility of the pearlescent printing scheme with inks prepared by pulverizing the solid elements in that the pearlescent properties will be lost if the pearlescent pigment particles are pulverized to an appropriately jettable size. To preserve the variable reflectivity of the pearlescent finish, or to make the finished surfaces pearlesce, it is easily understood that a larger reflecting surface for each of the constituent particles is preferred. In fact those of skill in the art recognize that, in order to preserve the pearlescent phenomenon in a printed image, the solid pearlescent pigment particles need to be on the order of ten of microns in diameter, each representing a little shiny mirror in the ink.
A desire to form variable digitally produced pearlescent images on successive image receiving medium substrates conflicts with the practical need in preparing jettable inks to pulverize the solids into as small a size as possible for those constituent elements of the jettable inks. When this requirement for pulverization is combined with a concern that only limited amounts of any pigment solids can be added to the jettable ink solution without adversely affecting the viscosity of the jettable ink solution, it becomes clear that producing high quality digital pearlescent images in a conventional digital printing process using jettable inks is nearly impossible. These limitations that may keep the advantages of pearlescent image forming for printed documents from being fully realized and exploited.